<PAGE>

                                                                    EXHIBIT 10.5


                      PREFERRED STOCK PURCHASE AGREEMENT

     This Preferred Stock Purchase Agreement (the "Agreement") is made as of
April 19, 2001, by and between Genaera Corporation, a Delaware corporation with
its principal place of business located at 5110 Campus Drive, Plymouth Meeting,
Pennsylvania, 19462 (the "Company"), and MedImmune, Inc., a Delaware corporation
with its principal place of business located at 35 West Watkins Mill Road,
Gaithersburg, Maryland 20878 (the "Purchaser").

                                  WITNESSETH:

     WHEREAS, the Company and the Purchaser are entering into a Collaboration
and License Agreement in the form attached hereto as Exhibit A (the
"Collaboration and License Agreement" and together with this Agreement, the
"Agreements").

     WHEREAS, in connection therewith, the Purchaser has agreed to purchase and
the Company has agreed to sell up to ten thousand (10,000) shares of the
Company's Series B Convertible Preferred Stock, par value $.001 per share, on
the terms and conditions set forth herein.

     NOW, THEREFORE, in consideration of the foregoing recitals and for other
good and valuable consideration, the adequacy of which is hereby affirmed, the
parties, intending to be legally bound, hereby agree as follows.

     1.   Authorization and Issuance.
          ---------------------------

          1.1  Authorization of Shares.  Subject to the terms and conditions of
               -----------------------
this Agreement, the Company has authorized the sale and issuance of up to ten
thousand (10,000) shares (the "Shares") of its Series B Convertible Preferred
Stock, par value $.001 per share (the "Series B Preferred Stock"), having the
rights, preferences, privileges and restrictions set forth in the Certificate of
Designations, Preferences and Rights of Series B Preferred Stock in
substantially the form attached hereto as Exhibit B (the "Certificate of
Designations").

          1.2  Issuance and Purchase of Shares. Subject to the terms and
               -------------------------------
conditions hereof, the Company will sell to the Purchaser, and the Purchaser
will buy from the Company, ten thousand (10,000) shares of its Series B
Preferred Stock at a purchase price per share of $1,000 (the "Per Share Price").
In consideration for the Shares, Purchaser shall pay the Company in United
States dollars the aggregate purchase price of ten million dollars
($10,000,000).

     2.   The Closing. The completion of the purchase and sale of the Shares
          -----------
(the "Closing") shall occur within three (3) business days following the
satisfaction of all of the conditions precedent in Sections 3.1 and 3.2 herein,
at the offices of Morgan, Lewis & Bockius LLP, 1701 Market Street, Philadelphia,
PA 19103, or such other time or place as may be agreed upon by the Company and
the Purchaser (the "Closing Date"). At the Closing, the Company shall deliver to
the Purchaser one or more stock certificates for the Shares, registered in the
Purchaser's name, representing the ten thousand (10,000) Shares to be purchased
by the Purchaser against payment of the purchase price therefor, by wire
transfer in immediately available funds, per the Company's instructions.
<PAGE>

     3.   Conditions to Closing.
          ---------------------

          3.1  Obligation of Purchaser.  The obligation of the Purchaser to
               -----------------------
purchase and pay for the Shares at the Closing is subject to the satisfaction as
of the Closing of the following conditions, any one or more of which may be
waived by the Purchaser:

               (i)    the accuracy in all material respects of the
          representations and warranties made by the Company herein and the
          fulfillment in all material respects of those undertakings of the
          Company to be fulfilled prior to the Closing;

               (ii)   the purchase of the Shares by the Purchaser hereunder
          shall not be prohibited by any applicable law or governmental rule or
          regulation;

               (iii)  the 180-day notice of termination period as set forth in
          Section 13.2(a) of the License and Collaboration Agreement dated April
          28, 2000 between the Company and Genentech, Inc. (the "Genentech
          Agreement") shall have expired on June 13, 2001, or the Company and
          Genentech, Inc. shall have agreed to such earlier termination of said
          agreement;

               (iv)   the Company and the Purchaser shall have entered into the
          Collaboration and License Agreement;

               (v)    the Certificate of Designations shall have been filed in
          the office of the Secretary of State of the State of Delaware;

               (vi)   the Company shall have caused the delivery to the
          Purchaser of stock certificates representing the Purchaser's ownership
          of the Shares;

               (vii)  the Purchaser shall have received from Morgan, Lewis &
          Bockius LLP, counsel to the Company, an opinion addressed to the
          Purchaser in a form agreed to by the parties; and

               (viii) the Company shall have obtained all necessary "Blue Sky"
          law permits and qualifications, or have the availability of exemptions
          therefrom, required by any state for the offer and sale of the Shares
          and the underlying Conversion Shares (as defined herein).

          3.2  Obligation of Company.  The Company's obligation to complete the
               ---------------------
purchase and sale of the Shares and deliver the stock certificate(s) to the
Purchaser at the Closing shall be subject to the following conditions, any one
or more of which may be waived by the Company:

               (i)  receipt by the Company by wire transfer in immediately
          available funds of $10,000,000, representing the full amount of the
          purchase price for the Shares being purchased hereunder;

                                       2
<PAGE>

               (ii)   the accuracy in all material respects of the
          representations and warranties made by the Purchaser and the
          fulfillment in all material respects of those undertakings of the
          Purchaser to be fulfilled prior to the Closing;

               (iii)  the 180-day notice of termination period as set forth in
          Section 13.2(a) of the Genentech Agreement shall have expired on June
          15, 2001, or the Company and Genentech, Inc. shall have agreed to such
          earlier termination of said agreement;

               (iv)   the Company and the Purchaser shall have entered into the
          Collaboration and License Agreement; and

               (v)    the Company shall have obtained all necessary "Blue Sky"
          law permits and qualifications, or have the availability of exemptions
          therefrom, required by any state for the offer and sale of the Shares
          and the underlying Conversion Shares.

     4.   Representations and Warranties of the Company.  Except as set forth on
          ---------------------------------------------
Schedule 1 attached hereto, the Company represents and warrants to Purchaser as
of the date hereof as follows:

          4.1  Organization and Standing.  The Company has been duly
               -------------------------
incorporated and is validly existing as a corporation in good standing under the
laws of the State of Delaware and has all requisite corporate power and
authority to conduct its business as described in the Company's Annual Report on
Form 10-K for the year ended December 31, 2000 filed with the Securities and
Exchange Commission (the "SEC").  The Company is qualified to do business as a
foreign corporation in the Commonwealth of Pennsylvania and there are no other
jurisdictions in which its ownership or lease of property or conduct of its
business requires such qualification, except where the failure to be so
organized or in good standing would not be reasonably likely to have a material
adverse effect on the business, financial condition or results of operations of
the Company.

          4.2  Authorized and Issued Capital Stock.  The authorized capital
               -----------------------------------
stock of the Company, upon the filing of the Certificate of Designations, will
consist of (i) 45,000,000 shares of common stock, par value $.002 per share (the
"Common Stock"), of which 32,543,886 shares are outstanding, and (ii) 9,211,031
shares of preferred stock, $.001 par value per share (the "Preferred Stock"), of
which (a) 80,000 shares have been designated "Series A Convertible Preferred
Stock," and of which 1,188 shares are issued and outstanding as of the date
hereof, and (b) 10,000 shares have been designated "Series B Convertible
Preferred Stock." The Series B Preferred Stock shall have the rights,
preferences, privileges and restrictions set forth in the Certificate of
Designations. Immediately prior to the Closing, there are no outstanding shares
of Series B Preferred Stock. All of the outstanding shares of Common Stock and
Preferred Stock were duly authorized and validly issued and are fully paid and
non-assessable shares. The Company has reserved 10,000 shares of Series B
Preferred Stock for issuance hereunder and 2,000,000 shares of Common Stock for
issuance upon conversion of the Series B Preferred Stock (the "Conversion
Shares"). Except as disclosed in the SEC Documents (as defined herein), there

                                       3
<PAGE>

are no options, warrants or other rights to purchase or acquire any of the
Company's authorized and unissued capital stock.

          4.3  Due Authorization, Execution, Delivery and Performance of the
               -------------------------------------------------------------
Agreements.  The execution, delivery and performance of the Agreements by the
----------
Company, including, but not limited to, the issuance and sale of the Shares and
the issuance of the Conversion Shares in accordance with the provisions of the
Certificate of Designations, (i) have been duly authorized by all requisite
corporate action by the Company, including by its directors and shareholders to
the extent required under the Delaware General Corporation Law, and (ii) will
not violate (a) any law or the Certificate of Incorporation, as amended (the
"Certificate of Incorporation") or the Amended and Restated Bylaws (the
"Bylaws") of the Company, or (b) any provision of any indenture, mortgage,
agreement, contract or other instrument to which the Company is a party or by
which the Company or any of its properties or assets is bound as of the date
hereof, or result in a breach of or constitute (upon notice or lapse of time or
both) a default under any such indenture, mortgage, agreement, contract or other
instrument or result in the creation or imposition of any lien, security
interest, mortgage, pledge, charge or other encumbrance, of any material nature
whatsoever, upon any properties or assets of the Company, except where any such
breach, default, lien or encumbrance would not be reasonably likely to have a
material adverse effect on the business, financial condition or results of
operations of the Company. The Company has no subsidiaries and does not own or
control any equity interest in any corporation, association or other business
entity. Upon their execution and delivery, and assuming the valid execution
thereof by the Purchaser, the Agreements will constitute valid and binding
obligations of the Company, enforceable against the Company in accordance with
their respective terms, except as enforceability may be limited by applicable
bankruptcy, insolvency, reorganization, moratorium or similar laws affecting
creditors' and contracting parties' rights generally and except as
enforceability may be subject to general principles of equity (regardless of
whether such enforceability is considered in a proceeding in equity or at law)
and except as the indemnification and contribution agreements of the Company in
Section 6.3 hereof may be legally unenforceable.

          4.4  Issuance, Sale and Delivery of the Shares and the Conversion
               ------------------------------------------------------------
Shares.  When issued and paid for, the Shares to be sold hereunder by the
------
Company will be validly issued and outstanding, fully paid and non-assessable,
free and clear of any liens, charges, claims or other encumbrances. When issued
in compliance with the provisions of the Certificate of Designation, the
Conversion Shares will be validly issued and outstanding, fully paid and non-
assessable, free and clear of any liens, charges, claims or other encumbrances.
The Shares and the Conversion Shares shall not entitle the holders of the
outstanding capital stock of the Company to preemptive, anti-dilutive or other
rights to subscribe to or acquire Common Stock or other securities of the
Company; provided, however, that the Shares and the Conversion Shares will be
         --------  -------
subject to restrictions on transfer under state or federal securities laws and
restrictions set forth in Section 6 hereof.

          4.5  Exemption from Registration.  Assuming the accuracy of the
               ---------------------------
Purchaser's representations and warranties set forth in Section 5 hereof, the
offer, issuance and sale of the Shares pursuant to this Agreement will be exempt
from the registration and prospectus delivery requirements of the Securities Act
of 1933, as amended (together with the rules and regulations promulgated
thereunder, the "Securities Act").

                                       4
<PAGE>

          4.6  Intellectual Property.  The Company has title and ownership of,
               ---------------------
or rights to use, all patents, trademarks, service marks, trade names,
copyrights, trade secrets, information, proprietary rights and processes
necessary for its business as now conducted without any conflict with or
infringement of the rights of others, except as would not have a material
adverse effect on the Company. Except as set forth in Schedule 1 hereof and
except for the Ludwig License Agreements (as such term is defined in the
Collaboration and License Agreement), there are no outstanding options,
licenses, or agreements of any kind relating to the subject of the Collaboration
and License Agreement and the transactions and the research contemplated by the
Collaboration and License Agreement, nor is the Company bound by or a party to
any material options, licenses or agreements of any kind with respect to the
patents, trademarks, service marks, trade names, copyrights, trade secrets,
licenses, information, proprietary rights and processes of any other person or
entity. The Company has not received any communications alleging that the
Company has violated or, by conducting its business as proposed, would violate
any of the patents, trademarks, service marks, trade names, copyrights or trade
proprietary rights of any other person or entity and that management reasonably
believes will have a material adverse effect on the Company. To the best
knowledge of the Company, no other person or entity is infringing or violating
any of the patents, trademarks, service marks, trade names, copyrights, trade
secrets, information, proprietary rights and processes necessary for the
Company's business as now conducted. To the best knowledge of the Company, none
of its employees is obligated under any contract (including licenses, covenants
or commitments of any nature) or other agreement, or subject to any judgment,
decree or order of any court or administrative agency, that would interfere with
the use of his or her best efforts to promote the interests of the Company or
that would conflict with the Company's business as proposed to be conducted.
Neither the execution nor delivery of this Agreement, nor the carrying on of the
Company's business by the employees of the Company, nor the conduct of the
Company's business as proposed, will, to the best of the Company's knowledge,
conflict with or result in a breach of the terms, conditions or provisions of,
or constitute a default under, any contract, covenant or instrument under which
the Company or any of such employees is now obligated except as would not have a
material adverse effect on the Company. The Company does not believe it is or
will be necessary to utilize any inventions of any of its employees (or people
it currently intends to hire) made prior to their employment by the Company.

          4.7  Litigation.  There are no actions, suits, proceedings or
               ----------
investigations pending or, to the Company's knowledge, threatened against the
Company or its properties before any court or governmental agency which
management believes is likely to have a material adverse effect on the Company.
The foregoing includes, without limitation, actions pending or threatened
involving the prior employment of any of the Company's employees, their use in
connection with the Company's business of any information or techniques
allegedly proprietary to any of their former employers, or their obligations
under any agreements with prior employers. The Company is not a party or subject
to the provisions of any order, writ, injunction, judgment or decree of any
court or government agency or instrumentality. There is no action, suit,
proceeding or investigation by the Company currently pending or which the
Company intends to initiate.

          4.8  Material Agreements.  All of the contracts and agreements to
               -------------------
which the Company is a party and which are in effect as of December 31, 2000 and
which the Company considers to be material are listed as exhibits to the
Company's Form 10-K for the period ended

                                       5
<PAGE>

December 31, 2000. Any contracts and agreements to which the Company became a
party between January 1, 2001 and the date hereof and which the Company
considers to be material are set forth on Schedule 1 hereto. All such contracts
and agreements are valid, binding and in full force and effect in all material
respects, subject to laws of general application relating to bankruptcy,
insolvency and the relief of debtors and rules of law governing specific
performance, injunctive relief or other equitable remedies, and the Company has
not received any indication of an intention to terminate any such contract or
agreement by any of the parties to any such contract or agreement.

          4.9  Additional Information.  The Company represents and warrants that
               ----------------------
the information contained in the following documents (the "SEC Documents"),
copies of which have been furnished to the Purchaser, was true and correct in
all material respects and did not contain any untrue statement of a material
fact or omit to state any material fact necessary to make the statements
contained therein in light of the circumstances under which they were made, not
misleading as of their respective dates:

               (i)  the Company's 2000 annual report to stockholders;

               (ii) the Company's annual report on Form 10-K for the year ended
          December 31, 2000 (without exhibits);

               (iv) the notice of annual meeting of stockholders and proxy
          statement for the Company's 2001 annual meeting of stockholders to be
          held on May 16, 2001; and

               (v)  all other documents, if any, filed by the Company with the
          SEC since December 31, 2000, pursuant to the reporting requirements of
          the Exchange Act.

As of its filing date, each SEC Document complied as to form in all material
respects with the applicable requirements of the Securities Act and the Exchange
Act (as defined below), as the case may be.

          4.10 Financial Statements.  The audited financial statements and
               --------------------
unaudited interim financial statements of the Company included in the SEC
Documents fairly present, in all material respects, in conformity with GAAP
applied on a consistent basis (except as may be indicated in the notes thereto),
the financial position of the Company as of the dates thereof and its results of
operations and cash flows for the periods then ended (subject to normal year-end
adjustments and to the omission of certain footnotes required by GAAP in the
case of any unaudited interim financial statements). The Company has no
liabilities or obligations, absolute or contingent, that are required by GAAP to
be set forth in such financials, that have not been so set forth.

          4.11 Compliance with Other Instruments, None Burdensome, Etc.  The
               -------------------------------------------------------
Company is not in violation of: (i) any term of its Certificate of
Incorporation, or its Bylaws; (ii) in any material respect, any term or
provision of any mortgage, indebtedness, indenture, contract, agreement,
instrument, judgment or decree; and (iii) to the best of its knowledge, any
order, statute, rule or regulation applicable to the Company except for such
violations that would not,

                                       6
<PAGE>

individually or in the aggregate, have a material adverse effect. The execution,
delivery and performance of and compliance with this Agreement, and the issuance
of the Shares and the Conversion Shares, have not resulted and will not result
in any violation of, or conflict with, or constitute a default under, the
Company's Certificate of Incorporation or its Bylaws, and have not and will not
result in any material violation of, or conflict with, or constitute a material
default under, any of its agreements nor result in the creation of any mortgage,
pledge, lien, encumbrance or charge upon any of the properties or assets of the
Company.

          4.12  Registration Rights.  Other than the rights granted under
                -------------------
Section 6 of this Agreement, the rights granted pursuant to the Registration
Rights Agreement dated April 28, 2000 by and between the Company and Genentech,
Inc. and the rights granted to certain investors in connection with the
Company's August 2000 private placement, the Company has not granted or agreed
to grant any registration rights, including piggyback rights, to any person or
entity except for rights that the Company believes have expired.

          4.13  Governmental Regulations, Etc.  Except as required pursuant to
                -----------------------------
the Securities Act, the Securities Exchange Act of 1934 (the "Exchange Act"), as
amended, and state securities laws, the Company is not subject to any federal or
state law or regulation limiting its ability to execute and deliver this
Agreement, or to offer, sell or issue the Shares or the Conversion Shares, or to
consummate any other transaction contemplated hereby, other than (i) the filing
of the Certificate of Designations in the office of the Secretary of State of
the State of Delaware prior to the Closing and (ii) the qualification (or taking
of such action as may be necessary to secure an exemption from qualification, if
available) of the offer and sale of the Shares and the Conversion Shares under
applicable "Blue Sky" laws, which filings and qualifications, if required, will
be accomplished in a timely manner

          4.14  No Material Change.  Except as specifically set forth in this
                ------------------
Agreement or in the SEC Documents, as of the date hereof since December 31,
2000: (a) the Company has not entered into any transaction that was not in the
ordinary course of business; (b) there has been no material adverse change in
the condition (financial or otherwise) of the business, prospects, property,
assets or liabilities of the Company, other than in the ordinary course of
business; (c) there has been no damage to, destruction of or loss of physical
property (whether or not covered by insurance) materially and adversely
affecting the assets, financial condition, operating results, prospects,
business or operations of the Company; (d) the Company has not declared or paid
any dividend or made any distribution on its stock, or redeemed or purchased or
otherwise acquired any of its stock or incurred any material tax liability; (e)
the Company has not changed any compensation arrangement or agreement with any
of its key employees or executive officers, or changed the rate of pay of its
employees as a group, other than in the ordinary course of business; (f) the
Company has not received notice that there has been a loss of a material
customer of the Company; (g) the Company has not changed or amended any contract
by which the Company or any of its assets are bound or subject that would have a
material adverse effect on the Company; (h) there has been no resignation or
termination of employment of any officer or key employee of the Company, and the
Company does not know of any impending resignation or termination of employment
of any such officer or key employee that if consummated would have a material
adverse effect on the Company; (i) there has been no labor dispute involving the
Company or any of its employees and, to the knowledge of the Company and none is
pending or threatened; (j) there has been no material and adverse change in the
contingent obligations of the

                                       7
<PAGE>

Company (nor in any material contingent obligation of the Company regarding any
director, stockholder or key employee or officer of the Company) by way of
guaranty, endorsement, indemnity, warranty or otherwise; (k) there have been no
loans or guarantees in excess of $15,000 made by the Company to any of its
employees, officers or directors other than travel advances and other advances
made in the ordinary course of business; (l) there has been no waiver by the
Company of a valuable right or of a debt owing to the Company that would have a
material adverse effect on the Company; (m) there has not been any satisfaction
or discharge of any material lien, claim or encumbrance or any payment of any
material obligation by the Company except in the ordinary course of business and
that is not material to the assets, properties, financial condition, operating
results or business of the Company; (n) the Company has not sold or transferred
any patent, service mark, trade name, copyright, trade secret or proprietary
right necessary for its business; and (o) there has been no other event or
condition of any character that would have a material adverse effect on the
Company.

          4.15  Listing of Common Stock.  The Conversion Shares will, upon
                -----------------------
issuance, be listed on the Nasdaq National Market.

          4.16  Registration on Form S-3.  The Company meets the registrant
                ------------------------
eligibility requirements for use of Form S-3 for registration of securities
under the Securities Act.

          4.17  Disclosure.  No representation or warranty made by the Company
                ----------
in this Agreement, any schedule or exhibit hereto, or any certificate delivered
hereunder, when taken together, contains any untrue statement of a material
fact, or omits to state a material fact necessary to make the statements
contained herein or therein not misleading in light of the circumstances under
which they were made.

     5.   Representations and Warranties of the Purchaser.
          -----------------------------------------------

          5.1  The Purchaser represents and warrants to, and covenants with, the
Company that: (i) the Purchaser, taking into account the personnel and resources
it can practically bring to bear on the purchase of the Shares contemplated
hereby, is knowledgeable, sophisticated and experienced in making, and is
qualified to make, decisions with respect to investments in equity securities
presenting an investment decision like that involved in the purchase of the
Shares, including investments in equity securities issued by early-stage
biotechnology companies such as the Company; (ii) the Purchaser or its counsel,
accountants or other investment advisors have requested, received, reviewed and
considered (or the Company has made available to them) all information requested
by them in connection with the Purchaser's decision to purchase the Shares;
(iii) the Purchaser is acquiring the Shares for its own account, for investment
only and with no present intention of distributing any of the Shares, in
violation of the Securities Act, or any arrangement or understanding with any
other persons regarding the distribution of such Shares, in violation of the
Securities Act; (iv) the Purchaser has completed or caused to be completed the
Registration Statement Questionnaire, attached hereto as Appendix I, for use in
                                                         ----------
preparation of a Registration Statement (as defined herein) and the answers
thereto are true, correct and complete in all material respects as of the date
hereof and will be true, correct and complete in all material respects as of the
effective date of a Registration Statement; (v) the Purchaser has, in connection
with its decision to purchase the Shares, relied solely upon the SEC Documents
and the representations and warranties of the

                                       8
<PAGE>

Company contained herein, as well as any investigation of the Company completed
by the Purchaser or its counsel, accountants or other investment advisers; (vi)
the Purchaser is an "accredited investor" within the meaning of Rule 501 of
Regulation D promulgated under the Securities Act; (vii) the address of the
Purchaser set forth on the preamble hereto is the Purchaser's principal business
address, upon which the Company may rely for the purpose of complying with
applicable "Blue Sky" laws; and (viii) the Purchaser is able to bear the
economic risk of an investment in the Shares.

          5.2  The Purchaser will not, directly or indirectly, offer, sell,
pledge, transfer or otherwise dispose of (or solicit any offers to buy, purchase
or otherwise acquire or take a pledge of) any of the Shares or the Conversion
Shares, except in compliance with the Securities Act, and the rules and
regulations promulgated thereunder and by the provisions of Section 6 hereof.
Prior to conversion of all the Shares into the Conversion Shares in accordance
with the Certificate of Designations, Purchaser represents that it will not have
any short positions in the Company's Common Stock and will not deliver
Conversion Shares to cover any short positions taken prior to such conversion of
all the Shares. The Purchaser hereby covenants not to make any sale of the
Conversion Shares under a Registration Statement without effectively causing the
prospectus delivery requirement under the Securities Act to be satisfied, and
the Purchaser acknowledges and agrees that such Conversion Shares that are to be
resold by Purchaser pursuant to a Registration Statement are not transferable on
the books of the Company unless the certificate submitted to the transfer agent
evidencing the Conversion Shares is accompanied by a separate officer's
certificate: (i) in the form of Appendix II hereto, (ii) executed by an officer
                                -----------
of the Purchaser or by an authorized person designated by the Purchaser, and
(iii) to the effect that (A) the Conversion Shares have been sold pursuant to
and in accordance with the Registration Statement and (B) the requirement of
delivering a current prospectus has been satisfied.  The Purchaser acknowledges
that there occasionally may be times when the Company must, subject to the
limitations set forth below in this Section 5.2 and in Section 6.1(viii)(F)
herein, suspend the use of the prospectus forming a part of a Registration
Statement until such time as an amendment to such Registration Statement has
been filed by the Company and declared effective by the Securities and Exchange
Commission, or until such time as the Company has filed an appropriate report
with the Commission pursuant to the Exchange Act.  The Purchaser hereby
covenants that it will not sell any Conversion Shares pursuant to said
prospectus during the period commencing at the time at which the Company gives
the Purchaser notice of the suspension of the use of said prospectus and ending
at the time the Company gives the Purchaser notice that the Purchaser may
thereafter effect sales pursuant to said prospectus.  The Company shall only be
able to suspend the use of said prospectus for two periods aggregating no more
than ninety (90) business days in any twelve-month period, of which no
individual period shall be longer than forty-five (45) consecutive days;
provided, however, that in no event shall the aggregate number of days pursuant
to which the Company may suspend the use of a prospectus pursuant to this
Section 5.2 and delay an underwritten public offering pursuant to Section
6.1(viii)(F) herein exceed one hundred twenty (120) days in any twelve-month
period.  The Purchaser further covenants to notify promptly the Company of the
sale of all of its Conversion Shares.

          5.3  The Purchaser understands that although there currently is a
public market for its Common Stock, Rule 144 promulgated under the Act ("Rule
144") requires, among other conditions, a one-year holding period prior to the
resale (in limited amounts) of securities

                                       9
<PAGE>

acquired in a non-public offering without having to satisfy the registration
requirements under the Act. The Purchaser understands and hereby acknowledges
that the Company is under no obligation to register the Shares or the Conversion
Shares under the Act or any state securities or "Blue Sky" laws other than as
set forth in Section 6 hereof. The Purchaser agrees to hold the Company and its
directors, officers, employees, controlling persons and agents and their
respective heirs, representatives, successors and assigns harmless and to
indemnify them against all liabilities, costs and expenses incurred by them as a
result of (i) any misrepresentation made by the Purchaser contained in this
Agreement (including Appendix I and Appendix II herein), (ii) any sale or
                     ----------     -----------
distribution by the Purchaser in violation of the Act or any applicable state
securities or "Blue Sky" laws or (iii) any untrue statement of a material fact
made by the Purchaser and contained herein.

          5.4  The Purchaser hereby consents to and understands that the Shares
shall bear a restrictive legend in substantially the following form (and a stop-
transfer order may be placed against transfer of the certificates for such
securities):

          "The Shares represented by this certificate have been
          acquired for investment and have not been registered
          under the Securities Act of 1933. The Shares may not be
          sold or transferred in the absence of such registration
          or exemption therefrom.

          The Shares represented by this certificate and the
          rights of holders thereof are subject to certain
          restrictions on transfer and other restrictions, and
          the holder of the Shares represented by this
          certificate (including any holders) are bound by the
          terms of restrictions set forth in the Preferred Stock
          Purchase Agreement between the original Purchaser and
          the Company (copies of which may be obtained from the
          Company)."

          5.5  The Purchaser further represents and warrants to, and covenants
with, the Company that (i) the Purchaser has full right, power, authority and
capacity to enter into the Agreements and to consummate the transactions
contemplated thereby and has taken all necessary action to authorize the
execution, delivery and performance of the Agreements, and (ii) upon the
execution and delivery of the Agreements, the Agreements shall constitute a
valid and binding obligation of the Purchaser enforceable in accordance with
their terms, except as enforceability may be limited by applicable bankruptcy,
insolvency, reorganization, moratorium or similar laws affecting creditors' and
contracting parties' rights generally and except as enforceability may be
subject to general principles of equity (regardless of whether such
enforceability is considered in a proceeding in equity or at law).

          5.6  No representation or warranty made by the Purchaser in this
Agreement, any schedule or exhibit hereto, or any certificate delivered
hereunder, when taken together, contains any untrue statement of a material
fact, or omits to state a material fact necessary to make the statements
contained herein or therein not misleading in light of the circumstances under
which they were made.

     6.   Registration of Shares.
          ----------------------

                                       10
<PAGE>

          6.1  Registration Procedures and Expenses.
               ------------------------------------

               (i)   The Company shall as soon as practicable, but in no event
          later than May 31, 2001, prepare and file with the Commission a
          Registration Statement on Form S-3 (or if such form is unavailable to
          the Company, on such other form deemed appropriate for the
          registration of the Common Stock by the Commission) (the "Initial
          Registration Statement") to register up to two million (2,000,000)
          shares of the Conversion Shares for resale by the Purchaser
          ("Registrable Securities") and such Initial Registration Statement
          shall include the Plan of Distribution section set forth in Appendix I
                                                                      ----------
          hereof, and shall use commercially reasonable efforts to cause the
          Initial Registration Statement to become effective as soon as
          practicable thereafter; provided that the Company shall be obligated
          to include in the Initial Registration Statement or the Subsequent
          Registration Statement (as defined herein), as the case may be, the
          disposition of Conversion Shares held by a Record Transferee (as
          defined herein) only if the Purchaser transferring any Conversion
          Shares to a third party (the "Record Transferee") gives notice of such
          transfer to the Company and any such Record Transferee agrees in
          writing to be bound by the provisions of Section 7 hereof and this
          Section 6; if the Purchaser converts the Shares into Conversion Shares
          at any time following the fifth anniversary of the date hereof and the
          number of such Conversion Shares is in excess of two million
          Conversion Shares, the Company shall as soon as practicable following
          such conversion prepare and file with the Commission a second
          Registration Statement to register such excess Conversion Shares for
          resale by the Purchaser (the "Subsequent Registration Statement,"
          together with the Initial Registration Statement, the "Registration
          Statements" and, individually, a "Registration Statement"), and shall
          use commercially reasonable efforts to cause the Subsequent
          Registration Statement to become effective as soon as practicable
          thereafter;

               (ii)  The Company shall promptly prepare and file with the
          Commission such amendments and supplements to a Registration Statement
          and the prospectus used in connection therewith as may be necessary to
          keep such Registration Statement effective until the first to occur of
          (a) such date when either all of the Registrable Securities have been
          sold pursuant thereto or, by reason of Rule 144(k) of the Commission
          under the Securities Act or any other rule of similar effect, the
          Registrable Securities are no longer required to be registered for the
          sale thereof by the Purchasers in ordinary market transactions without
          imposition of any volume, manner of sale or other limitations, or (b)
          the second anniversary of the Closing Date (the "Registration
          Period");

               (iii) The Company shall promptly furnish to the Purchaser such
          number of copies of prospectuses in conformity with the requirements
          of the Securities Act and such other documents as the Purchaser may
          reasonably request, in order to facilitate the public sale or other
          disposition of all or any of the Registrable Securities by the
          Purchaser;

                                       11
<PAGE>

               (iv)   The Company shall promptly file documents required of the
          Company for normal "Blue Sky" clearance for the disposition by the
          Purchaser and any Record Transferees of the Registrable Securities in
          such states as the Purchaser may reasonably request in writing to the
          Company, provided however, that the Company shall not be required (i)
          to qualify to do business or consent to service of process in any
          jurisdiction in which it is not now so qualified or has not so
          consented; (ii) subject itself to general taxation in any such
          jurisdiction; (iii) provide any undertakings that cause the Company
          undue expense or burden; or (iv) make any change in its charter or
          bylaws;

               (v)    The Company shall promptly inform the Purchaser when any
          stop order has been issued with respect to a Registration Statement
          and use commercially reasonable efforts to promptly cause such stop
          order to be withdrawn;

               (vi)   Except with respect to an underwritten offering as set
          forth in Section 6.1(viii) hereof, the Company shall bear all expenses
          in connection with the procedures in paragraphs (i) through (v) of
          this Section 6.1 and the registration of the Registrable Securities,
          other than fees and expenses, if any, of counsel or other advisers to
          the Purchaser or the other Purchasers and any expenses relating to the
          sale of the Registrable Securities by the Purchasers (including
          without limitation, broker's commissions, discounts or fees of any
          nature and transfer taxes or changes of any nature); and

               (vii)  A Questionnaire related to the Registration Statements to
          be completed by the Purchaser, including the Purchaser's approval of
          the "Plan of Distribution" section of the Registration Statement is
          attached as Appendix I to this Agreement.  The Company may amend the
                      ----------
          Plan of Distribution section as necessary to comply with the
          requirements of the Commission, provided, that, the Company shall send
                                          --------  ----
          any such amendments to the Purchaser at least two (2) business days
          prior to the filing of the Registration Statement with the Commission
          for the Purchaser's approval, which shall not be unreasonably withheld
          and, in the absence of written objection prior to filing, shall be
          deemed to be an approval by the Purchaser of such amended section.

               (viii) Recognizing that an underwritten public offering of the
          Conversion Shares may be mutually beneficial to the Company and the
          Purchaser, the Company agrees that if the Purchaser determines to sell
          the Conversion Shares by means of an underwritten public offering, the
          Company agrees to cooperate with the Purchaser and the underwriter to
          effectuate an underwritten offering, including entering into an
          underwriting agreement, subject to the following conditions and
          limitations:

                      (A)  the Purchaser shall only be permitted to effect the
          resale of Conversion Shares through one (1) underwritten offering;

                                       12
<PAGE>

                    (B)  the underwriter selected by the Purchaser shall be
          reasonably acceptable to the Company and the Company shall not be
          required to execute an underwriting agreement unless and until it
          determines in its reasonable judgment that the terms of such
          underwriting agreement are customary for transactions of that size and
          type;

                    (C)  the Company shall have the ability to include shares of
          Common Stock to be sold on its behalf in the offering subject to
          approval by the underwriters; if the total amount of shares of Common
          Stock  requested by the Company to be included in the offering exceeds
          the amount of securities that the underwriters determine is compatible
          with the success of the offering, then only such number of shares of
          Common Stock to be sold on the Company's behalf shall be included as
          the underwriters determine will not jeopardize the success of the
          offering;

                    (D)  the registration of the Conversion Shares to be offered
          in the underwriting shall be on the Registration Statement;

                    (E)  the Purchaser shall not be permitted to resell
          Conversion Shares pursuant to an underwritten offering until it has
          converted all of its Shares and unless such offering is for at least
          75% of the aggregate Conversion Shares issued to Purchaser pursuant to
          all of its conversions of Shares;

                    (F)  the Company shall be permitted to delay an underwritten
          offering requested by Purchaser once in any twelve month period for a
          period not to exceed ninety (90) days if the Board of Directors of the
          Company determines that it would be detrimental to effect such an
          underwritten offering at such time; provided, however, that in no
          event shall the aggregate number of days pursuant to which the Company
          may suspend the use of a prospectus pursuant to Section 5.2 herein and
          delay an underwritten public offering pursuant to this Section
          6.1(viii)(F) exceed one hundred twenty (120) days in any twelve-month
          period;

          In connection with an underwritten offering of the Conversion Shares
          under this Section 6.1(viii), (i) Purchaser and Company agree to
          equally split any and all incremental accounting fees and expenses and
          printing costs attributable solely to the underwritten offering, (ii)
          Purchaser agrees to bear any and all discounts, concessions,
          commissions or other compensation to be received by the underwriters,
          and (iii) each party hereto agrees to bear their respective counsel
          fees and expenses.

          6.2  Transfer of Shares After Registration.  The Purchaser agrees that
               -------------------------------------
it will not effect any disposition of the Shares that would constitute a sale
within the meaning of the Securities Act except as contemplated in a
Registration Statement or as otherwise in compliance with applicable securities
laws and that it will promptly notify the Company of any material changes in the
information set forth in a Registration Statement regarding the Purchaser or its
plan of distribution. The Company agrees to notify each Purchaser whose
Registrable Securities are registered on a Registration Statement and its agent
at any time when a prospectus relating to

                                       13
<PAGE>

any Registrable Securities covered by such Registration Statement is required to
be delivered under the Securities Act of the happening of any event as a result
of which the prospectus included in such registration statement, as then in
effect, includes an untrue statement of a material fact or omits to state a
material fact required to be stated therein or necessary to make the statements
therein not misleading in light of the circumstances then existing, and to
promptly file such amendments and supplements as may be necessary so that, as
thereafter delivered to such Purchasers of such Registrable Securities, such
prospectus shall not include an untrue statement of a material fact or omit to
state a material fact required to be stated therein or necessary to make the
statements therein not misleading in light of the circumstances then existing
and use commercially reasonable efforts to cause each such amendment and
supplement to become effective.

          6.3  Indemnification and Contribution.  For the purpose of this
               --------------------------------
Section 6.3, the term "Selling Stockholder" shall include the Purchaser, its
officers, directors, and/or trustees and any affiliate or controlling person of
such Purchaser or any permitted assign hereunder; and the term "untrue
statement" shall include any untrue statement or alleged untrue statement, or
any omission or alleged omission to state in a Registration Statement a material
fact required to be stated therein or necessary to make the statements therein,
in the light of the circumstances under which they were made, not misleading.

               (i)  The Company agrees to indemnify and hold harmless each
          Selling Stockholder from and against any losses, claims, damages or
          liabilities to which such Selling Stockholder may become subject
          (under the Securities Act or otherwise) insofar as such losses,
          claims, damages or liabilities (or actions or proceedings in respect
          thereof) arise out of, or are based upon, any breach of the
          representations set forth in Section 4 hereof by the Company, or any
          untrue statement of a material fact contained in a Registration
          Statement, or arise out of any failure by the Company to fulfill any
          agreement, covenant or undertaking contained in this Agreement or
          included in a Registration Statement, and the Company will reimburse
          such Selling Stockholder for any reasonable legal or other expenses
          reasonably incurred in investigating, defending or preparing to defend
          any such action, proceeding or claim; provided, however, that the
                                                --------  -------
          Company shall not be liable in any such case if such loss, claim,
          damage or liability arises out of, or is based upon, (i) an untrue
          statement made in a Registration Statement in reliance upon and in
          conformity with written information furnished to the Company by or on
          behalf of such Selling Stockholder specifically for use in preparation
          of a Registration Statement (which shall be deemed to include the
          information set forth in the Registration Statement Questionnaire and
          in the plan of distribution section of the prospectus), (ii) the
          failure of such Selling Stockholder to comply with the covenants and
          agreements contained herein respecting transfer or sale of the Shares,
          including, but not limited to, the Purchaser's covenant in Section 5.2
          hereof to comply with the prospectus delivery requirements under the
          Securities Act, or (iii) any statement or omission in any Prospectus
          that is corrected in any subsequent Prospectus that was delivered to
          the Purchaser prior to the pertinent sale or sales by the Purchaser.
          The Company shall also not be liable for amounts paid in settlement of
          any loss, claim, damage or liability if such settlement is effected
          without the prior written

                                       14
<PAGE>

          consent of the Company, which consent shall not be unreasonably
          withheld or delayed.

               (ii)  The Purchaser agrees to indemnify and hold harmless the
          Company (and each person, if any, who controls the Company within the
          meaning of Section 15 of the Securities Act, each officer of the
          Company who signs the Registration Statement and each director of the
          Company) from and against any losses, claims, damages or liabilities
          to which the Company (or any such officer, director or controlling
          person) may become subject (under the Securities Act or otherwise),
          insofar as such losses, claims, damages or liabilities (or actions or
          proceedings in respect thereof) arise out of, or are based upon, any
          breach of the representations set forth in Section 5 hereof by the
          Purchaser, or any failure by the Purchaser to comply with the
          covenants and agreements contained herein relating to transfer or sale
          of the Shares, including, but not limited to, the Purchaser's covenant
          in Section 5.2 hereof to comply with the prospectus delivery
          requirements under the Securities Act,  or any untrue statement of a
          material fact contained in a Registration Statement if such untrue
          statement was made in reliance upon and in conformity with written
          information furnished by or on behalf of the Purchaser specifically
          for use in a Registration Statement.  The Purchaser will reimburse the
          Company (or such officer, director or controlling person), as the case
          may be, for any legal or other expenses reasonably incurred in
          investigating, defending or preparing to defend any such action,
          proceeding or claim.  The Purchaser agrees that the information
          regarding the Purchaser or its officers, directors and affiliates and
          their intended plan of distribution of the Conversion Shares set forth
          in a Registration Statement questionnaire, the form of which is
          attached as Appendix I, or included from time to time in a
                      ----------
          Registration Statement (including without limitation the plan of
          distribution section of such Registration Statement) shall be deemed
          to be written information furnished to the Company by or on behalf of
          the Purchaser specifically for use in preparation of a Registration
          Statement.  The foregoing indemnification shall be limited in amount
          to the aggregate purchase price paid by the Purchaser hereunder.  The
          Purchaser shall also not be liable for amounts paid in settlement of
          any loss, claim, damage or liability if such settlement is effected
          without the prior written consent of the Purchaser, which consent
          shall not be unreasonably withheld or delayed.

               (iii) Promptly after receipt by any indemnified person of a
          notice of a claim or the beginning of any action in respect of which
          indemnity is to be sought against an indemnifying person pursuant to
          this Section 6.3, such indemnified person shall notify the
          indemnifying person in writing of such claim or of the commencement of
          such action; provided, however, that any failure by an indemnified
                       --------  -------
          person to notify an indemnifying person shall not relieve the
          indemnifying person from its obligations hereunder except to the
          extent that the indemnifying person is prejudiced thereby.  Subject to
          the provisions hereinafter stated, in case any such action shall be
          brought against an indemnified person and such indemnifying person
          shall have been notified thereof, such indemnifying person shall be
          entitled to participate therein, and, to the extent it shall wish, to
          assume and control the defense thereof, with counsel reasonably
          satisfactory to

                                       15
<PAGE>

          such indemnified person. After notice from the indemnifying person to
          such indemnified person of its election to assume the defense thereof,
          such indemnifying person shall not be liable to such indemnified
          person for any legal expenses subsequently incurred by such
          indemnified person in connection with the defense thereof; provided,
                                                                     --------
          however, that if there exists a conflict of interest that would make
          -------
          it inappropriate, in the opinion of counsel to the indemnifying
          person, for the same counsel to represent both the indemnified person
          and such indemnifying person or any affiliate or associate thereof,
          the indemnified person shall be entitled to retain its own counsel at
          the expense of such indemnifying person; provided further, however,
                                                   -------- -------  -------
          that no indemnifying person shall be responsible for the fees and
          expenses of more than one separate counsel for all indemnified parties
          hereunder and under the other Agreements.

               (iv) If the indemnification provided for in this Section 6.3 from
          the indemnifying person would be applicable by its terms but is
          otherwise unavailable, as determined by a court of applicable
          jurisdiction, to an indemnified person hereunder in respect of any
          losses, claims, damages, liabilities or expenses referred to herein,
          then the indemnifying person, in lieu of indemnifying such indemnified
          person, shall contribute to the amount paid or payable by such
          indemnified person as a result of such losses, claims, damages,
          liabilities or expenses in such proportion as is appropriate to
          reflect the relative fault of the indemnifying person and indemnified
          persons in connection with the actions which resulted in such losses,
          claims, damages, liabilities or expenses, as well as any other
          relevant equitable considerations.  The relative fault of such
          indemnifying person and indemnified persons shall be determined by
          reference to, among other things, whether any action in question,
          including any untrue or alleged untrue statement of a material fact,
          has been made by, or relates to information supplied by, such
          indemnifying person or indemnified persons, and the parties' relative
          intent, knowledge, access to information and opportunity to correct or
          prevent such action.  The amount paid or payable by a party as a
          result of the losses, claims, damages, liabilities and expenses
          referred to above shall be deemed to include, subject to the
          limitations set forth in this Section 6.3, any reasonable legal or
          other fees or expenses reasonably incurred by such party in connection
          with any investigation or proceeding.

               (v)  The parties hereto agree that it would not be just and
          equitable if contribution pursuant to this Section 6.3 were determined
          by pro rata allocation or by any other method of allocation which does
          not take account of the equitable considerations referred to in the
          immediately preceding paragraph.  Notwithstanding the provisions of
          this Section 6.3, Purchaser shall not be required to contribute any
          amount in excess of the dollar amount of the proceeds received by
          Purchaser upon the sale of the Conversion Shares, giving rise to such
          contribution obligation.  No person guilty of fraudulent
          misrepresentation (within the meaning of Section 11(f) of the
          Securities Act) shall be entitled to contribution from any person who
          was not guilty of such fraudulent misrepresentation.

                                       16
<PAGE>

               (vi) The indemnification and contribution provided in this
          Section 6.3 shall be in addition to any other rights and remedies
          available at law or in equity.

     7.   Compliance.  With a view to making available to Purchaser the benefits
          ----------
of Rule 144 promulgated under the Securities Act, the Company agrees:

          7.1  So long as Purchaser or any subsequent holder(s) of the Shares,
or the Conversion Shares, own the Shares, or the Conversion Shares, promptly
upon request, to furnish to such owner such information as may be necessary and
otherwise reasonable to cooperate with such owner to permit such owner to sell
the Conversion Shares pursuant to Rule 144 without registration.

          7.2  If at any time the Company is not required to file reports with
the SEC pursuant to Sections 13 or 15(d) of the Exchange Act, to use
commercially reasonable efforts to, upon the request of Purchaser or any
subsequent holder(s) of any of the Shares or the Conversion Shares, to make
publicly available other information so long as is necessary to permit
publication by brokers and dealers of quotations for the Common Stock and sales
of the Conversion Shares in accordance with Rule 15c2-11 under the Exchange Act.

     8.   Miscellaneous.
          -------------

          8.1  Governing Law. This Agreement shall be governed in all respects
               -------------
by the laws of the Commonwealth of Pennsylvania as such laws are applied to
agreements between Pennsylvania residents entered into and performed entirely in
Pennsylvania and without regard to conflicts of law.

          8.2  Survival of Warranties. The representations and warranties of the
               ----------------------
Company and Purchaser contained in or made pursuant to this Agreement shall
survive until one year following the execution of this Agreement, the payment of
the aggregate purchase price and the delivery of certificate(s) evidencing the
Shares.

          8.3  Successors and Assigns.  The provisions hereof shall inure to the
               ----------------------
benefit of, and be binding upon, the successors, assigns, heirs, executors and
administrators of the parties hereto (specifically including successors in
interest to the Shares).

          8.4  Entire Agreement; Amendments.  This Agreement sets forth all of
               ----------------------------
the promises, covenants, agreements, conditions and undertakings between the
parties hereto with respect to the subject matter hereof, and supersedes all
prior and contemporaneous agreements and understandings, inducements or
conditions, express or implied, oral or written, except as contained herein.
This Agreement may not be changed orally but only by an agreement in writing,
duly executed by or on behalf of the party or parties again whom enforcement of
any waiver, change, modification, consent or discharge is sought.

          8.5  Notices, Etc.  All notices and other communications required or
               -------------
permitted hereunder shall be effective upon receipt and shall be in writing and
may be delivered in person, by facsimile, overnight delivery service or U.S.
mail, in which event it may be mailed by first-class, certified or registered,
postage prepaid, addressed

                                       17
<PAGE>

                    if to the Company, to:

                    Genaera Corporation
                    5110 Campus Drive
                    Plymouth Meeting, PA 19462
                    Attention: Roy C. Levitt, M.D.
                    Facsimile: (610) 941-5399

                    with a copy so mailed to:

                    Morgan, Lewis & Bockius LLP
                    1701 Market Street
                    Philadelphia, Pennsylvania 19103-2921
                    Attention: Randall B. Sunberg
                    Facsimile: (215) 963-5299

                    if to the Purchaser, to:

                    MedImmune, Inc.
                    35 W. Watkins Mill Road
                    Gaithersburg, M.D. 20878
                    Attn:  David M. Mott
                    Facsimile: (301) 527-4000

                    with a copy so mailed to:

                    Dewey Ballantine LLP
                    1301 Avenue of the Americas
                    New York, New York 10019
                    Attention: Frederick W. Kanner
                    Facsimile: (212) 259-6333

or to such other person at such other place as the either party shall designate
to the other in writing;

          8.6  Severability of This Agreement. If any provision of this
               ------------------------------
Agreement shall be judicially determined to be invalid, illegal or
unenforceable, the validity, legality and enforceable of the remaining
provisions shall not in any way be affected or impaired thereby.

          8.7  Titles and Subtitles. The titles of the paragraphs and
               --------------------
subparagraphs of this Agreement are for convenience of reference only and are
not to be considered in construing this Agreement.

          8.8  Counterparts. This Agreement may be executed in any number of
               ------------
counterparts, each of which shall be an original, but all of which together
shall constitute one instrument.

                                       18
<PAGE>

                 [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

                                       19
<PAGE>

     IN WITNESS WHEREOF, the parties have caused this Agreement to be duly
executed and delivered by their proper and duly authorized officers as of the
day and year first written above.

GENAERA CORPORATION

By: /s/ Roy C. Levitt, M.D.
    -----------------------------------------
Name:  Roy C. Levitt, M.D.
Title: President and Chief Executive Officer


MEDIMMUNE, INC.


By: /s/ David M. Mott
    -----------------------------------------
Name:  David M. Mott
Title:  Chief Executive Officer

                                       20
<PAGE>

                                                                      APPENDIX I


                              GENAERA CORPORATION

                     REGISTRATION STATEMENT QUESTIONNAIRE
                     ------------------------------------

          In connection with the preparation of the Registration Statement,
please provide us with the following information:

          1.   Pursuant to the "Selling Stockholder" section of the Registration
Statement, please state your organization's name exactly as it should appear in
the Registration Statement: ___________________________________________________.

          2.   Please provide the number of shares of Common Stock that your
organization will own beneficially or of record immediately after Closing
(indicating whether you have sole or shared voting or dispositive power over
such securities as determined under applicable rules of the Securities and
Exchange Commission):

<TABLE>
<CAPTION>
        VOTING POWER                DISPOSITIVE POWER
Sole  Shared                  Sole  Shared
<S>           <C>             <C>             <C>             <C>
__________    __________      __________      __________      Common Stock beneficially owned
                                                              prior to the date hereof

__________    __________      __________      __________      Conversion Shares issuable upon
                                                              conversion of the Shares

__________    __________      __________      __________      Common Stock issuable upon exercise
                                                              of options or warrants that you may
                                                              own, to the extent such shares of
                                                              Common Stock are deemed to be
                                                              beneficially owned

TOTAL:        ________________ shares of Common Stock beneficially owned after Closing
</TABLE>

                                      I-1
<PAGE>

          3.   Has your organization had any position, office or other material
relationship within the past three years with the Company or its affiliates
other than as disclosed in the Company's proxy statement for its 2000 annual
meeting of stockholders?

               _____ Yes  _____ No

          If yes, please indicate the nature of any such relationships below:

          _________________________________________________________________

          _________________________________________________________________

          _________________________________________________________________

          4.   Attached is a draft of the proposed "Plan of Distribution"
section of the prospectus included in the Registration Statement.  Please
confirm that the draft is a correct and complete statement of your intended plan
of distribution.

               _____ Yes  _____ No


MEDIMMUNE, INC.

By: __________________________________
Name:
Title:

                                      I-2
<PAGE>

                             PLAN OF DISTRIBUTION



          To the extent required, the names of any agent or broker-dealer and
applicable commissions or discounts and any other required information with
respect to any particular offer will be set forth here or in the accompanying
prospectus supplement.  The selling stockholders reserve the sole right to
accept or reject, in whole or in part, any proposed purchase of the shares to be
made directly or through agents.

          The selling stockholder may from time to time, in one or more
transactions, sell all or a portion of the shares on the Nasdaq National Market,
in negotiated transactions, in underwritten transactions or otherwise, at prices
then prevailing or related to the then current market price or at negotiated
prices.  The offering price of the shares from time to time will be determined
by the selling stockholder and, at the time of such determination, may be higher
or lower than the market price of the shares on the Nasdaq National Market.  In
connection with an underwritten offering, underwriters or agents may receive
compensation in the form of discounts, concessions or commissions from a selling
shareholder or from purchasers of shares for whom they may act as agents, and
underwriters may sell shares to or through dealers, and such dealers may receive
compensation in the form of discounts, concessions or commissions from the
underwriters and/or commissions from the purchasers for whom they may act as
agents.  Under agreements that may be entered into by us, underwriters, dealers
and agents who participate in the distribution of shares may be entitled to
indemnification by us against certain liabilities, including liabilities under
the Securities Act, or to contribution with respect to payments which such
underwriters, dealers or agents may be required to make in respect thereof.  The
shares may be sold directly or through broker-dealers acting as principal or
agent, or pursuant to a distribution by one or more underwriters on a firm
commitment or best-efforts basis.  The methods by which the shares may be sold
include:

 .    a block trade in which the broker-dealer so engaged will attempt to sell
     the shares as agent but may position and resell a portion of the block as
     principal to facilitate the transaction;

 .    purchases by a broker-dealer as principal and resale by such broker-dealer
     for its account pursuant to this prospectus;

 .    ordinary brokerage transactions and transactions in which the broker
     solicits purchasers;

 .    privately negotiated transactions; and

 .    underwritten transactions.

          The selling stockholder and any underwriters, dealers or agents
participating in the distribution of the offered shares may be deemed to be
"underwriters" within the meaning of the Securities Act, and any profit on the
sale of the shares by the selling stockholder and any commissions received by
these broker-dealers may be deemed to be underwriting commissions under the
Securities Act.

                                      I-3
<PAGE>

     When the selling stockholder elects to make a particular offer of shares, a
prospectus supplement, if required, will be distributed which will identify any
underwriters, dealers or agents and any discounts, commissions and other terms
constituting compensation from the selling stockholder and any other required
information.

     In order to comply with the securities laws of certain states, if
applicable, the shares may be sold only through registered or licensed brokers
or dealers.  In addition, in certain states, the shares may not be sold unless
they have been registered or qualified for sale in such state or an exemption
from such registration or qualification requirement is available and is complied
with.

     We have agreed to pay all costs and expenses incurred in connection with
the registration under the Securities Act of the shares, including, without
limitation, all registration and filing fees, fees and disbursements of our
counsel and certain accounting and printing costs.  The selling stockholder will
pay any brokerage fees and commissions, fees and disbursements of legal counsel
for the selling stockholder and stock transfer and other taxes attributable to
the sale of the shares.  We also have agreed to indemnify the selling
stockholder and its respective officers, directors and trustees and each person
who "controls," within the meaning of the Securities Act, the selling
stockholder against certain losses, claims, damages, liabilities and expenses
arising under the securities laws in connection with this offering.  The selling
stockholder has agreed to indemnify us, our officers and directors and each
person who "controls" Genaera Corporation against any losses, claims, damages,
liabilities and expenses arising under the securities laws in connection with
this offering with respect to written information furnished to us by such
selling stockholder.

                                      I-4
<PAGE>

                                                                     APPENDIX II

Attention:

                  PURCHASER'S CERTIFICATE OF SUBSEQUENT SALE
                  ------------------------------------------

     The undersigned, [an officer of, or other person duly authorized by]
___________________________________________________ [fill in official name of
individual or institution] hereby certifies that [he/she/said institution] is
the Purchaser of the shares evidenced by the attached certificate, and as such,
sold such shares on  [date] pursuant to and in accordance with registration
statement number ____________________ [fill in the number of or otherwise
identify registration statement] and the requirement of delivering a current
prospectus by the Company has been complied with in connection with such sale.

Print or Type:

Name of Purchaser
      (individual or
      institution):    _______________________________

Name of individual
representing
Purchaser (if an
institution)    _______________________________

Title of individual
representing
Purchaser (if an
institution):      _______________________________


Signature by:

Individual purchaser
or individual repre-
senting purchaser:     _______________________________

                                     II-1
<PAGE>

                                   EXHIBIT A

                      COLLABORATION AND LICENSE AGREEMENT

                       [Included as Exhibit 10.4 to this
                        Quarterly Report on Form 10-Q]
<PAGE>

                                   EXHIBIT B

                          CERTIFICATE OF DESIGNATIONS

                       [Included as Exhibit 3.7 to this
                        Quarterly Report on Form 10-Q]